Mechanism of demyelination and remyelination are studied in mice infected with mouse hepatitis virus (MHV) and Theiler's mouse poliovirus. Lesions induced by these viruses are examined by electron microscopy and autoradiography. The incorporation of viral antigen in myelin lamellae is searched for during the early stage of infection of oligodendrocytes by MHV. Modifications of the course of autoimmune demyelination in Theiler's mouse poliovirus infection is investigated by desensitization and immunosuppression. The replication of oligodendrocytes following demyelination after infection with MHV and Theiler's virus is studied by autoradiography.